1. Field of the Invention
The present invention relates to an optical disc apparatus that is used for recording and reproducing information on an optical recording medium. In particular, the present invention relates to an optical disc apparatus equipped with a liquid crystal element.
2. Description of Related Art
Optical recording media including a compact disc (hereinafter referred to as a CD) and a digital versatile disc (hereinafter referred to as a DVD) are widely available. In addition, researches on the high density of the optical recording medium are being carried on in recent years in order to increase recording capacity of the optical recording medium. For example, a high density optical recording medium such as a Blu-Ray Disc (hereinafter referred to as a BD) is being available in the market. Reproducing and recording information on the optical recording medium is performed by using an optical disc apparatus that is equipped with an optical pickup that projects a light beam from a light source onto the optical recording medium so that information is read or written.
An optical recording medium is provided with a transparent protective layer having a predetermined thickness that is formed so as to cover a recording surface for protecting the same. It is difficult to form this protective layer so as to have a predetermined thickness uniformly in the manufacturing process, and actually the thickness may have some error. If an error is generated in the thickness of the protective layer of the optical recording medium, spherical aberration is generated, which causes a problem of deterioration in a reproduced signal that is obtained when the optical recording medium is written and read by the optical disc apparatus.
This spherical aberration increases in proportion to biquadrate of a numerical aperture (NA) of an objective lens that condenses a light beam from a light source of the optical pickup onto the recording surface of the optical recording medium. Therefore, generation of the spherical aberration due to a variation of thickness of the protective layer will become a serious problem particularly in the optical disc apparatus that supports a BD or the like, because it generally uses the objective lens having a high numerical aperture (for example, NA=0.85).
There is conventionally known an optical disc apparatus that is equipped with a liquid crystal element having a refractive index that is variable in accordance with a drive voltage, which is disposed in an optical system of the optical pickup of the optical disc apparatus for a purpose of correcting spherical aberration. A voltage that is applied to the liquid crystal element is controlled so that a phase of a light beam that passes through the liquid crystal element for correcting the spherical aberration. It is necessary to give a condition for correcting spherical aberration (i.e., a drive voltage for driving the liquid crystal element) to the liquid crystal element every time when an optical recording medium is loaded to the optical disc apparatus, so as to correct spherical aberration that is generated by a variation of the optical recording medium (e.g., a variation of thickness of the protective layer).
As such a technique, for example, Japanese patent publication No. 3489193 discloses an optical disc apparatus that can correct spherical aberration by using an optimal correction value constantly during its reproducing operation even if spherical aberration is generated due to a thickness error or the like of a protective layer of an optical recording medium. It is noted that a tracking servo gain is decreased in accordance with a degree of the spherical aberration if the spherical aberration is generated in the optical system. Therefore, the optical disc apparatus is structured so that the correction value of spherical aberration is transferred if necessary in the direction in which the tracking servo gain increases.
However, since the optical disc apparatus disclosed in the Japanese patent publication No. 3489193 has the structure in which a drive voltage of the liquid crystal element is changed at least two times or more for obtaining the optimal spherical aberration, it takes much time to obtain an optimal value for correcting spherical aberration. Thus, there is a problem that it takes long time until reproduction with a good quality can be performed.